1. Field of the Invention
The invention is directed to a thin film transistor and a manufacturing method thereof and more particularly, to a thin film transistor having a semiconductor layer with better mobility and a manufacturing method thereof.
2. Description of Related Art
FIG. 1 is a schematic diagram showing a pixel structure of the related art. Referring to FIG. 1, a conventional pixel structure 1 includes a thin film transistor 10 and a pixel electrode 20. The thin film transistor 10 is disposed on a substrate 2 and includes a gate 11, a gate insulation layer 12, a source 13, a semiconductor layer 14, a dielectric layer 15, a drain 16 and a protecting layer 17. The gate 11 is disposed on the substrate 2, and the gate insulation layer 12 covers the gate 11 and the substrate 2. The source 13 is disposed on the gate insulation layer 12. The semiconductor layer 14 is disposed above the gate 11, extends from the gate insulation layer 12 to the source 13. The dielectric layer 15 covers the source 13, the gate insulation layer 12 and the semiconductor layer 14 and has a first contact window 15a. The drain 16 is disposed on the dielectric layer 15 and contacts the semiconductor layer 14 through the first contact window 15a. The protecting layer 17 covers the dielectric layer 15 and the drain 16 and has a second contact window 17a, and the pixel electrode 20 contacts the drain 16 through the second contact window 17a. 
As shown in FIG. 1, during a process of manufacturing the pixel structure 1, the source 13 is manufactured before the semiconductor layer 14 is formed, and the drain 16 is manufactured after the semiconductor layer 14 is formed, and thus, the semiconductor layer 14 does not contact etchant used for patterning the source 13 and the drain 16 and has good thin film quality. Additionally, in the pixel structure 1, the source 13 is separated from the drain 16 with the semiconductor layer 14 disposed therebetween, and thus, a distance between the source 13 and the drain 16 can be shortened, so as to improve element characteristics (e.g., an aperture ratio, a display resolution and so on) of the thin film transistor 10.
However, in the pixel structure 1, part of the semiconductor layer 14 (e.g., region X shown in FIG. 1) is above the source 13. When a turn-on voltage (i.e., a high voltage) is applied to the gate 11, the gate 11, theoretically, generates an electric field to turn on the semiconductor layer 14. However, due to part of the electric field being shielded by the source 13, the part of the semiconductor layer 14 (i.e., the region X) distributed on the source 13 is affected by the shielding effect and may hardly be turned on. As a result, mobility of the semiconductor layer 14 of the thin film transistor 10 becomes poor.
Accordingly, how to improve the issue of poor mobility of the semiconductor layer 14 in the thin film transistor 10 has become one of the problems to be solved.